Auto injectors are well-known in the art, and are often preferred by users for self administration of medicine, such as for subcutaneous injection of medicines such as insulin, medicine to treat or alleviate multiple sclerosis, rheum, lupus, etc. or for emergency injection of e.g. adrenaline or epinephrine, such as injection in to muscle tissue.
The needles used for injection subcutaneously and for injection into muscle tissue are typically of different lengths. Typically, needles used for subcutaneous injections are about 12 mm (“half-inch”) whereas needles used for injection into muscle tissue may have a length of 20-25 mm (“inch”), to ensure that muscle tissue is reached.
The selected needle bore may also affect the degree of patient discomfort during injection. Smaller bore diameters, typically provide more patient comfort, whereas larger bore diameters enable more rapid delivery of the liquid through the needle and with a lower force. A compromise is therefore needed in selecting needle bore to provide acceptable patient comfort and liquid delivery through the needle characteristics.
Allergic reactions tend to become an ever increasing problem and for the treatment of severe allergic reactions (anaphylaxis) to foods, insect stings or bites, drugs and other allergens, as well as idiopathic or exercise induced anaphylaxis, adrenaline or epinephrine is typically used.
Epinephrine works quickly to reverse the symptoms of an anaphylactic reaction and epinephrine injected into the muscle of the front of the upper outer thigh is typically used for the emergency treatment of anaphylaxis.
Typically, epinephrine auto-injectors are single use injectors for injecting a pre-measured single dose of epinephrine for the emergency treatment of anaphylaxis.
However, when administering some drugs, such as epinephrine, a single dose may not be sufficient to treat the anaphylaxis. In order for one patient to be able to receive full treatment including one, two or more doses with a single injector, different auto injectors have been suggested.
Different possibilities for the injection of two doses from a same syringe have been suggested and in U.S. Pat. No. 7,927,303 and EP700307, two-dose auto injectors are disclosed allowing the automatic delivering of a first dose of a medicament and the manual refitting of the auto injector so that the once used syringe may be re-inserted into the auto injector for administration of a second dose.
In WO 2011/111006, an auto injector is disclosed in which the locking and releasing of the drive spring of the auto injector is controlled by providing stepped guide means with ramps for two successive slidings there along of slide means operated by the spring and connected with the syringe and relevant plunger. Thus, after a first dose has been delivered, a further dose may be delivered using the same spring and slide the syringe further along the slide means.
It is a disadvantage of the disclosed auto injector that the length of the device increases significantly when a second dose is delivered.
Furthermore, auto injectors have been disclosed focusing on reducing the risk of wet injection. For example, WO 2012/045827 discloses an auto injector having an arrangement for coupling a plunger rod to either a syringe or a stopper arranged in the syringe. However, it is a disadvantage of the auto injector as disclosed that a single compression spring is applied for injecting the needle and for injecting the medicament, in that the resistance of the needle penetrating the skin may tend to force the syringe backwards in a syringe carrier, whereby the stopper may contact the forward moving plunger rod and expelling the medicament prematurely which may result in a wet injection.
U.S. Pat. No. 7,785,292 discloses an auto injector comprising a housing wherein a single driving mechanism is used to insert the needle and to inject the medicament. The syringe is moved to a forward position before the piston is allowed to move forward to inject the medicament. A locking mechanism engages the drive with the syringe when the syringe is not in the forward position and engages the syringe with the housing when the syringe is in the forward position.
The prior art documents thus disclose the use of a single drive mechanism, the engagement of which is shifted depending on the position of the syringe.
There is therefore a need for an auto injector which allows for selectively delivery of one or more doses, wherein subsequent doses are also delivered automatically and which is both compact in size and highly robust with respect to the avoidance of wet injections.